Why Microsoft 365 is slow from China, and how to fix it

Companies buy Microsoft 365 Business and staff still complain that Outlook spins and Teams drops calls. The cause is usually not the subscription tier — it is the path the traffic takes.

1. How Microsoft front doors work

Microsoft 365 operates many regional front doors worldwide. Ideally a client connects to the nearest one and Microsoft's backbone carries the rest. Microsoft provides a connectivity test that reports the actual front door and latency in use.

The problem appears when the local path to the nearest front door is poor: DNS resolves to a more distant node, for example Hong Kong or Singapore, and traffic detours. The symptom is "it works, but slowly".

2. Why Teams is more sensitive than Outlook

Teams media uses UDP and is extremely sensitive to loss and jitter — 1% packet loss is enough to blur video and break audio. HTTP traffic (Outlook, SharePoint) retransmits and tolerates far more loss.

Many corporate networks block UDP by default or apply aggressive NAT timeouts, forcing Teams back to TCP and visibly degrading quality. Check this first.

3. Four optimisation paths

• DNS and resolution: ensure clients resolve to the nearest front door rather than a distant node via public DNS. Lowest cost — do this first.
• Dedicated or optimised transit: route office traffic to Microsoft front doors over a quality international path, avoiding congested gateways. Suitable for 50+ user offices.
• Local caching and split tunnelling: keep large OneDrive and SharePoint transfers on local cache nodes to reduce repeated cross-border transfers.
• Endpoint policy: allow UDP 3478–3481, remove unnecessary proxy chains, and stop forcing Teams media through an HTTP proxy.

4. What not to do

Do not deep-inspect and proxy all office traffic. Once Teams media is intercepted by a proxy, latency rises sharply and the proxy becomes the bottleneck. Bypassing Microsoft traffic directly works better than proxying everything.

5. How to verify improvement

Microsoft's network assessment tool scores each component. Run it before and after, and compare front door, latency and UDP availability — more reliable than a subjective impression.